1. Field of the Invention
The present invention relates to apparatus and a method for controlling temperatures within an enclosure and, more particularly, for using the velocity of heated and/or cooled fluid as the temperature controlling means.
2. Description of Related Art and Other Considerations
Stress screening of a product is associated with vibrating the product under conditions which enables any defects, which may be a result of the manufacturing process, to be found. In this way, devices or products, which have defects caused by variations in fabrication processes or workmanship, may be screened out of the production line before being shipped to a customer. Examples of vibration stress screening apparatus is disclosed in U.S. Pat. Nos. 4,181,025 through 4,181,029 and 4,735,089.
Environmental screening many times also includes the application of heat and/or cold to the product. This is the other major form of stress which helps the process of finding defects. In a typical method for thermal screening, the product is placed in an environmental control chamber whose temperature is ramped up and down to allow time for the temperature or temperatures of the product to attain the specified temperature limits. Such a stress screening temperature chamber is illustrated and described in a paper entitled "Stress Screening of Electronic Modules" published in the JOURNAL OF THE IES, March/April, 1990 on pages 60-65.
Such conventional temperature screening as described in the above publication is subject to several disadvantages. Heating or cooling of the product is slow, because of the relatively small temperature differential between the heating or cooling air and the product, and the poor heat transfer because, even when air is circulated within the chamber, the velocity of the air is low. Another problem is that, towards the end of the heating or cooling cycle where the temperature of the product approaches the air temperature, there is an increasing time interval for the product to attain its goal temperature. This slowing down is caused by the decreasing temperature differential between the air and the product; thus, the heat flow decreases almost to zero as the temperature difference between the product and the air decreases towards zero. Further, energy usage is inefficient because of the heating and cooling process of the recycled air and of the chamber itself. In addition to achieving the minimum and maximum temperatures, for effective stress to be generated within a product using a heating and/or cooling fluid, the temperature of the product must be made to change rapidly. In order to compensate for such problems as above discussed, large and expensive air conditioning installations are required. Accordingly, because of such high cost and long screening times involved with conventional temperature screening processes, most commercial manufacturers screen only with vibration; to combine heating with vibration many times is not an economical option.